Two stage process for hydrocarbon synthesis

ABSTRACT

Hydrocarbon synthesis is carried out in a two stage process wherein the pressure in the first stage is relatively higher and the pressure in the second stage is relatively lower and the second stage catalyst comprises cobalt on alumina.

FIELD OF THE INVENTION

This invention relates to a two stage hydrocarbon synthesis process andeliminates the need to drive the initial or first stage reaction to thehighest possible conversion. More particularly, this invention relatesto a hydrocarbon synthesis process where in a first stage adequatecommercial levels of carbon monoxide conversion are obtained andemploying in a second, lower pressure stage, a catalyst providing highcarbon monoxide conversion at such lower pressures. Still moreparticularly, the carbon monoxide conversion activity of the second,lower pressure stage is about as high, and preferably higher, than thecarbon monoxide conversion activity of the first stage catalyst atsecond stage pressures.

BACKGROUND

Hydrocarbon synthesis catalysts, also known as Fischer-Tropschcatalysts, have been studied widely and by various researchers in thepast sixty years. Preferred processes now usually employ cobalt orruthenium, cobalt and ruthenium, or promoted cobalt catalysts. Thecatalysts are supported on a variety of supports, but usually they aresupported on inorganic oxides, such as alumina, silica, titania,silicaalumina, and the like.

Promoters can be used to enhance the activity or stability of cobalt orruthenium catalysts and these promoters may vary. For example, rheniumhas been used to promote cobalt catalysts supported on either titania oralumina, see U.S. Pat. No. 4,568,663 and U.S. Pat. No. 4,801,573respectively. Supported ruthenium catalysts are also quite useful forhydrocarbon synthesis (see U.S. Pat. Nos. 4,477,595; 4,171,320; and4,042,614). Also, ruthenium and zirconium have been used to promotecobalt supported on silica (see U.S. Pat. Nos.4,088,671, 4,599,481, and4,681,867). Two stage hydrocarbon synthesis was disclosed in U.S. Pat.No. 4,443,561 relating to hydrogen:carbon monoxide ratios, but making nodifferentiation based on the pressure in each reaction stage. Suffice tostate that a variety of cobalt containing catalysts have been disclosedfor hydrocarbon synthesis processes operating over a relatively widepressure range. However, to achieve practical results from a hydrocarbonsynthesis process, in the sense of converting carbon monoxide andobtaining the maximum availability of desired products, usually C₅ +products, these processes are conducted in at least two seriallyconnected stages. That is, with the exception of liquid removal, thesecond stage operates on the products of the first stage at essentiallythe outlet pressure of the first stage.

Other two stage hydrocarbon synthesis processes have been reported inthe literature. For example, EPA159759, published Oct. 30, 1985 employsa first stage cobalt catalyst and a second stage catalyst having watergas shift activity, while EPA1121.951 published June 27, 1984 employs asecond stage catalyst with activity for converting olefinic oroxygenated products to isomeric hydrocarbons. Also, several e.g., U.S.Pat. Nos. 4,547,609, 4,279,830, and 4,159,995 use an iron based firststage catalyst for hydrocarbon synthesis and a second stage catalysthaving activity for aromatization. Also, U.S. Pat. No. 4,624,968 employsan iron based first stage catalyst for producing olefins and a secondstage catalyst for converting olefins to paraffins with additional COand hydrogen. All of these systems are based on dual function catalystsystems, that is, where the first stage catalyst is active for aspecific chemical reaction and the second stage catalyst is active for adifferent chemical reaction. However, none of these systems involve atwo stage process in which catalysts of essentially equivalentfunctionality are tailored to the specific operating conditions of eachstage.

Hydrocarbon synthesis processes are known to be plagued with severalproblems. Of these problems, obtaining high conversion and dissipatingheat are among the foremost. Since hydrocarbon synthesis is anexothermic reaction, heat must be removed from the reactor to avoid hotspots, catalyst deactivation, and loss of selectivity at highertemperatures. There is usually a preferred temperature range foroperating the process which leads to the optimum selectivity to desiredhigher hydrocarbon products. Lack of efficient heat removal can lead tomuch higher temperatures in the reactor which, while increasing carbonmonoxide conversion, severely debits the selectivity to preferred higherhydrocarbons. At the same time, increasing conversion generates moreheat and, thus, a greater burden on heat exchange facilities. Thus, thegoals of high conversion and efficient heat transfer tend to oppose eachother. To alleviate the problem, lower conversion in a first stage canbe accommodated, thereby, reducing the heat load in the first stage.However, this reduced conversion must be made up in the second stage.

Now, increasing pressure for a given reaction and catalyst systemgenerally increases carbon monoxide conversion in hydrocarbon synthesis.However, there is a pressure drop across the first stage reactor andachieving adequate conversion in a second stage can require intermediatecompression of the unreacted synthesis gas, an expensive step.

This invention takes advantage of the finding that cobalt on aluminacatalysts, specifically cobaltrhenium on alumina is a more activehydrocarbon synthesis catalyst at low pressures than other commonly usedcatalysts.

SUMMARY OF THE INVENTION

In accordance with this invention, a two stage hydrocarbon synthesisprocess is provided in which the second stage acts at about the outletpressure of the first stage and utilizes a non-shifting catalyst that isat least about as active or more active for CO conversion at secondstage pressure than the first stage catalyst at second stage reactionpressure.

In hydrocarbon synthesis processes CO conversion activity increases aspressure increases. However, the relative increase in CO conversion withincreasing pressure is not the same for every catalyst. Thus, relativelyhigh CO conversion levels are obtained with silica or titania atrelatively higher pressures, e.g., above about 10-12 atmospheres,preferably above about 15 atmospheres. The level of CO conversion setsthe pressure drop through a reactor. Thus, a 50% CO conversion translateto a 50% drop in total pressure. For example, a 2:1 (approximatelystoichiometric) hydrogen:CO mixture at 20 atmospheres and entering afirst stage with 60% CO conversion will exit that stage with 2:1hydrogen:CO at about 8 atmospheres or a 60% drop in pressure (inaccordance with Boyle's law). The use of a second stage cobaltcontaining catalyst, supported primarily on alumina, for second stageconversion makes use of the discovery that at relatively lowerpressures, e.g., under about 10-12 atmospheres, cobalt-alumina catalystsare more active for CO conversion than other supported cobalt containingcatalysts. Further, because a two stage process is employed, a lower COconversion level in the first stage can be tolerated and the need forthe highest possible CO conversion is obviated. Thus, lower costcatalysts with somewhat reduced CO conversion capability may now be usedin the first stage.

This invention is not meant to preclude the use of any particularsupported, cobalt or ruthenium containing catalyst in the first stage.While CO conversion activity for cobalt or ruthenium containingcatalysts supported by titania or silica may be higher than that foralumina at relatively higher pressures, this increase in activity is ona volumetric basis, that is, moles CO converted per hour per volume ofcatalyst (cc CO conv/hour/cc cat), and other factors can influence thechoice of first stage catalyst, e.g., cost and availability ofmaterials.

By virtue of this invention, the refiner now has much more flexibilityin processing schemes because the first stage catalyst need notnecessarily be the most active catalyst available, and claims for themost active catalyst vary widely among catalyst and process developers.

The finding that cobalt supported on alumina is more active than othercatalysts, particularly cobalt containing catalysts, at lower pressuresindicative of second stage pressures, minimizes and preferablyeliminates the need for recompression of the feed to the second stage toregain CO conversion activity. Some recompression may be desirable, butin order to achieve the benefits of this invention, the second stagepressure is such that the cobalt/alumina catalyst is of at least equalCO conversion activity and preferably greater CO conversion activitythan the first stage catalyst.

In hydrocarbon synthesis reactions, increasing temperature usuallyincreases CO conversion activity for a given catalyst, but can bedisadvantageous relative to product selectivity. That is, increasingtemperature to increase conversion may lead to increased amounts ofmethane or carbon dioxide which are undesirable products.

Consequently, a balance must be struck between productivity andselectivity. Because hydrocarbon synthesis is a highly exothermicprocess and cooling is required to prevent excessive temperatures, thesecond stage reaction is usually at or about the same temperature as thefirst stage reaction, although the temperature in each stage may bedifferent depending on the refiner's needs.

Aluminas come in a variety of different phases and virtually every phaseis useful in some form of catalysis. The surface area of alumina mayvary from a few square meters per gram, e.g., 1-2 m² /gm (BET), foralpha aluminas, and up to 500-600 m² /gm or higher (BET) for eta orgamma aluminas. For hydrocarbon synthesis, higher surface area aluminas,such as gamma alumina, are preferred. Because this material has such ahigh surface area relative to rutile phase titania, considerably morecobalt can be deposited on to alumina than on to titania, a greaternumber of cobalt sites can be achieved, and the cobalt dispersion isincreased. That is, with the higher surface area material there is lesstendency for one crystal of cobalt to fall on another crystal of cobaltresulting in little or no increase in dispersion even though the amountof cobalt is increased. For example, the cobalt loading on a titaniasupport may range from about 5-25 wt%, preferably 10-25 wt% andexcellent CO conversion will be obtained. However, the cobalt loading onan alumina support may range from about 5-60 wt%, preferably 5-45 wt%;see, for example, U.S. Pat. No. 4,801,573. Thus, by increasing thecobalt loading, a suitable cobalt-alumina catalyst having adequate COconversion activity for a first stage reaction may be produced.

First stage reaction conditions include temperatures ranging from about160° C. to about 290° C., preferably about 190° C.-260° C., morepreferably about 190° C.-230° C. While pressures may range from aboutatmospheric to about 600 psig, first stage pressures generally rangefrom about 150 psig to about 400 psig. Hydrogen to CO ratios may rangefrom about 0.5:1 to about 4:1, preferably about 1.7:1 to 2.5:1, and mostpreferably at or slightly below stoichiometric the stoichiometric ratiobeing about 2.1:1. Space velocity for fixed bed reactors may range fromabout 100 v/v/hr to 10000 v/v/hr, preferably 300 v/v/hr to 5000 v/v/hr.

Second stage reaction conditions will be similar to first stageconditions except that pressures will be reduced by the level ofconversion in the first stage and pressure drops of 40%-70%corresponding to 40-70% conversion through the first stage are notuncommon. Preferred conversion levels in the first stage are at leastabout 50%, preferably about 50-70%, preferably 60-70% with concomitantreductions in pressure. There may be some need for hydrogen or carbonmonoxide make up, and this can be accomplished easily.

The products of the first stage comprise C₂ +, preferably C₅ +hydrocarbons, methane, water, carbon dioxide and unconverted synthesisgas, hydrogen and carbon monoxide. At reactor outlet conditions, some ofthe product will condense, simply because of the pressure drop acrossthe reactor, and that product is removed from the stream entering thesecond reaction stage. It may or may not be desirable to causecondensation and removal of product, but it is preferable to remove thatmuch of the product that is liquid at first stage outlet conditions.

In a preferred method of operation the hydrogen:carbon monoxide ratio ineach stage is at or slightly below the stoichiometric ratio, that is,from about 1.7:1 to about 2.1:1. Operation at slightly below thestoichiometric ratio in the first stage will result in a slightly lowerhydrogen:carbon monoxide ratio in the second stage, absent make-uphydrogen. This operation is preferred in order to retain some CO partialpressure exiting the second stage. Thus, if the hydrogen:carbon monoxideratio is above about 2.1:1 in the first stage and no make up carbonmonoxide is used, the second stage will use up all of the remainingcarbon monoxide leading to catalyst deactivation and processinstability. Regardless of the operating conditions, the, the outlet ofthe second stage should have at least about 0.5 atmospheres CO partialpressure, preferably at least about 0.7 atmospheres CO partial pressure.

The first stage catalyst can be any catalyst that provides adequate COconversion activity at the pressures utilized in the first stage; thatis, relatively higher pressure, e.g., above about 10-12 atmospheres. Thesupported catalyst contains cobalt or ruthenium as the active metal andis preferably a cobalt containing catalyst. That is, the catalyst maycontain cobalt or cobalt and ruthenium.

Ruthenium, when used as the primary catalytic metal is present onamounts ranging from about 0.5 to 5.0 wt%. When used in conjunction withcobalt, small amounts of ruthenium are adequate to promote catalystactivity, e.g., weight ratios of 0.1:1 up to 1:1.

Suitable support materials are those inorganic oxides used for catalystsupports and, for example, reported in U.S. Pat. Nos. 4,171,320 and4,042,614, and generally Group IVB or Group VB metal oxides, andalumina. Preferred materials are alumina, silicaalumina, silica andtitania, or supports containing these materials primarily. Specificallypreferred are titania and alumina supports or supports containingprimarily titania or alumina, that is, at least 50 wt% titania oralumina, preferably at least 70 wt% titania or alumina, and titaniapreferably 80%+titania. When titania is employed, the rutile phase ismost preferred, and the rutile:anatase ratio is at least 2:3, preferablyat least 3:2, more preferably at least 4:1. The rutile/anatase ratio isdetermined by ASTM D 3720-78: Standard Test Method for Ratio of Anataseto Rutile in Titanium Dioxide Pigments by Use of X-Ray Diffraction.Titania can also be used with other oxides, such as alumina, zirconia,and silica in amounts ranging from about 0.1 wt% to about 20 wt%,preferably 0.5 to 10 wt%, most preferably 1 to 5 wt%.

Promoter metals may also be used, such as zirconium, titanium, rhenium,cerium, hafnium, and uranium. Hafnimum, rhenium, and cerium arepreferred, titania supports and rhenium is particularly preferred forpromoting cobalt on titania and alumina supports. When promoter metalsare used, the ratio of promoter to primary catalytic metal, e.g., cobaltor ruthenium, is at least about 0.05:1, preferably at least about 0.1:1to 1:1. Rhenium has been shown to enhance the activity of cobalt ontitania, ruthenium on titania, and cobalt on alumina; see U.S. Pat. Nos.4,568,663, 4,558,030, and 4,801,573, respectively. Any of these catalystare suitable first stage catalysts.

The second stage catalyst is an alumina supported, cobalt containingcatalyst. The support is primarily alumina, and preferably essentiallyall alumina. The catalytic metal is cobalt, in amounts previouslymentioned, and may also contain a rhenium promoter. At second stagetotal pressures, which can be at most 40%, usually about 50-60% of thefirst stage total pressure, cobalt on alumina has superior CO conversionactivity relative to non-alumina supported catalysts (that is, catalyststhat contain at least 50% of another support material, e.g., titania).

Alumina support materials may also incorporate oxides of the actinidesand lanthanides. These oxides are known to provide stability andincreased hydrocarbon yields in Fischer-Tropsch processes. The use ofthese materials is reported in U.S. Pat. Nos. 4,399,234, 4,605,680, and4,801,573. Suitable oxides are, for example Sc₂ O₃, Y₂ O₃, Ac₂ O₃, Pr₂O₃, PrO₂, Nd₂ O₃, Sm₂ O₃, Eu₂ O₃, Gd₂ O₃, Tb₂ O₃, Tb₄ O₇, Dy₂ O₃, Ho₂O₃, Er₂ O₃, Tm₂ O₃, Yb₂ O₃, Lm₂ O₃, UO₂, UO₃, U₃ O₈, UO₄.2H₂ O, and thelike. Preferred materials are Th₂ O₃, La₂ O₃, Ce₂ O₂, ZrO₂, HfO₂, andunseparated rare earth mixtures high in lanthanium, praseodynium, andneodymium. The most preferred is thoria. These materials are used inamounts of 0.1 to about 10 wt%, preferably about 0.5 to 5.0 wt%.

Thus, the invention makes use of the finding that cobalt on alumina ismore active for CO conversion at lower pressures, e.g., up to up about10-12 atmospheres, than other catalysts, particularly cobalt containingcatalysts, and that at higher pressures, e.g., above about 10-12atmospheres any one of several catalysts may be adequate, for any one ofseveral reasons. This finding is based on comparisons at constanttemperature, since temperature can effect CO conversion activity.

Catalyst preparation is in accordance with well-known techniquesdescribed in earlier mentioned patents and other relevant literature.The procedures given hereinbelow are illustrative.

The catalytically active metal, or metals, preferably cobalt or cobaltpromoted rhenium, can be dispersed upon a calcined support in a mannerwhich will distribute the metal, or metals, essentially uniformlythroughout the particles from the center outwardly, or essentially uponthe peripheral surface of the particle, preferably the latter when fixedbed processes are employed. In distributing the metal, or metals,uniformly throughout the support particles, e.g., the metal, or metals,can be deposited on the support particles from solution in preselectedamounts to provide the desired absolute amounts, and weight ratio of therespective metal, or metals. Suitably, e.g., cobalt, or cobalt andrhenium, are composited with the support by contacting the support witha solution of a cobalt-containing compound, or salt, or arhenium-containing compound, or salt, followed by impregnation of theother component. Optionally, the cobalt, or cobalt and rhenium can beco-impregnated upon the support. The cobalt used in the impregnation canbe any organometallic or inorganic compound which decomposes to givecobalt oxides upon calcination, or can be directly reduced to cobalt inflowing hydrogen, such as cobalt nitrate, acetate, acetylacetonate,naphthenate, carbonyl, or the like, the nitrate being preferred.Likewise the rhenium compound used in the impregnation can be anyorganometallic or inorganic compound which decomposes to give rheniumoxides upon calcination, or rhenium upon reduction, e.g., perrhenicacid, ammonium perrhenate and the like. The amount of impregnationsolution used should be sufficient to completely immerse the carrier,usually within the range from about 1 to 20 times of the carrier byvolume, depending on the metal, or metals, concentration in theimpregnation solution. The impregnation treatment can be carried outunder a wide range of conditions including ambient or elevatedtemperatures. On the other hand, the catalytically active cobaltcomponent is most preferably dispersed and supported upon the peripheralsurface of the support as a thin catalytically active surface layer, orfilm, ranging in average thickness from about 20 microns to about 250microns, preferably from about 40 microns to about 200 microns, with theloading of the cobalt expressed as the weight metallic cobalt per packedbulk volume of catalyst ranging from about 0.03 grams (g) per cubiccentimeter (cc) to about 0.15 g/cc, preferably from about 0.04 g/cc toabout 0.09 g/cc catalyst. The feature of a high cobalt metal loading ina thin catalytically active layer located at the surface of theparticles can optimize the activity, selectivity and productivity of thecatalyst in producing liquid hydrocarbons from synthesis gas, whileminimizing methane formation in fixed bed reactors.

The surface impregnated catalysts can be prepared by spray techniqueswhere a dilute solution of a cobalt compound, alone or in admixture witha promoter metal compound, or compounds, as a spray is repetitivelycontacted with hot support particles. The particulate support particlesare maintained at temperatures equal to or above about 140° C. whencontacted with the spray, and suitably the temperature of the supportparticles ranges from about 140° C. up to the decomposition temperatureof the cobalt compound, or compounds in admixture therewith; preferablyfrom about 140° C. to about 190° C. The cobalt compound employed in thesolution can be any organometallic or inorganic compound whichdecomposes to give cobalt oxide upon initial contact or uponcalcination, such as cobalt nitrate, cobalt acetate, cobaltacet:ylacetonate, cobalt naphthenate, cobalt carbonyl, or the like.Cobalt nitrate is especially preferred while cobalt halide and sulfatesalts should generally be avoided. The cobalt salts may be dissolved ina suitable solvent, e.g., water, organic or hydrocarbon solvent such asacetone, methanol, pentane or the like. The total amount of solutionused should be sufficient to supply the proper catalyst loading, withthe film being built up by repetitive contacts between the support andthe solvent. The preferred catalyst is one which comprises cobalt, orcobalt and promoter, dispersed upon the support. Suitably, the supportparticles are contacted with a spray which contains from about 0.05 g/mlto about 0.25 g/ml, preferably from about 0.10 g/ml to about 0.20 g/ml,of the cobalt compound or cobalt compound plus the compound containingthe promoter metal, generally from at least about 3 to about 12contacts, preferably from about 5 to about 8 contacts, with interveningdrying and calcination steps being required to form surface films of therequired thicknesses. The hot support particles, in other words, arespray-contacted in a first cycle which includes the spray contact oer sewith subsequent drying and calcination, a second cycle which includesper se with subsequent drying and calcination, etc., to form a film ofthe required thickness and composition. The drying steps are generallyconducted at temperatures ranging above about 20° C., preferably fromabout 20° C. to about 125° C., and the calcination steps at temperaturesranging above about 150 C, preferably from about 150° C. to about 500°C.

Alternatively, cobalt and ruthenium with or without promoters may beimpregnated onto the support by immersing the support in appropriatesolutions as described above and in relevant references known to theart.

In preparing catalyst some care need be exercised to insure that thecatalyst supports are treated appropriately. That is, they must beadequately inert and strong enough to withstand reaction conditions, andthey must not be treated in a manner as to detract from their catalyticactivity. For example, reducing the metal salts or oxides to theelemental metal on an alumina support conditions should be in the areaof about 350° C.-500° C. for about 10 hours, while titania is bettertreated at temperatures of about 200°-400 ° C. for 2-5 hours. For moreinformation on proper support treatment, see C. Bartholomew, R. Reuel,Ind. Eng. Chem. Prod. Res. Dev. 24, 56 (1985).

The following examples illustrate this invention.

Example 1 - Comparative Performance of Al₂ O₃ and TiO₂ Supported Cobaltat 1 atmosphere.

These examples are shown in European Patent Application 0 313 375,published Apr. 26, 1989.

    ______________________________________                                        Catalyst      % CO Conversion                                                 ______________________________________                                        12% Co/Al.sub.2 O.sub.3                                                                     12                                                              12% Co/TiO.sub.2 *                                                                          11                                                              12% Co/TiO.sub.2 **                                                                         11                                                              ______________________________________                                         *Support Calcined at 500° C.                                           **Support Calcined at 600° C.                                     

Conditions: H₂ pretreatment 1° C./minute to 350° C., hold for 10 hours,2/1 H₂ /CO, 1680 cm³ /g cat/hr feed rate, 10-30 hour on stream time.

This example shows a slight advantage for Co/Al₂ O₃ relative to Co/TiO₂at low pressures.

Example 2 - Comparative performance of Al₂ O₃ and TiO₂ Supported Cobaltat 1 atmosphere

These examples are shown in Vannice, M.A., J. Catalysis, 74, 199-202, p.(1982).

    ______________________________________                                        Catalyst       μmol CO/g Cobalt/sec                                        ______________________________________                                        2%       Co/Al.sub.2 O.sub.3                                                                     20                                                         1.5%     Co/TiO.sub.2                                                                            13                                                         ______________________________________                                    

Conditions: H₂ pretreatment at 450° C. for 1 hour, 3/1 H₂ /CO, <1 houron stream time.

Again, a slight advantage for Co/Al₂ O₃ is shown relative to Co/TiO₂.

Example 3 - Comparative Performance of Al₂ O₃ and TiO₂ Supported Cobaltat 1 atmosphere

These examples are taken from Niiyama, H., Pan-Pacific Synfuels Conf.,B-11 (1982).

    ______________________________________                                        Catalyst      % CO Conversion                                                 ______________________________________                                        5% Co/Al.sub.2 O.sub.3                                                                      5.6                                                             5% Co/TiO.sub.2                                                                             3.4                                                             ______________________________________                                    

Conditions: H₂ pretreatment at 400° C. for 1 hour, 3/1 H₂ /CO, 234° C.reaction temperature, 2-3 hours on stream time.

These examples show a substantial advantage for Co/Al₂ O₃ relative toCo/TiO₂ because sufficient amounts of metal are now used to promote thehydrocarbon synthesis reaction.

Example 4 - Comparative Performance of Al₂ O₃ and TiO₂ Supported Cobaltat 1 atmosphere

These examples are taken from Reuel, R. and Bartholomew, C., J.Catalysis, 85, 78-88, p. (1984).

    ______________________________________                                                 H.sub.2                                                                       Pretreatment                                                                    Temp    Time    Nco         Activity                               Catalyst   (°C.)                                                                          (Hr)    (×10.sup.2)                                                                   % D   (N × D)                          ______________________________________                                        10% Co/Al.sub.2 O.sub.3                                                                  375     20      1.3   9.9   12.9                                              525      2      6.4   6.7   42.9                                   10% Co/TiO.sub.2                                                                         400     16      4.1   4.5   18.5                                              525      2      N/A   2.3   N/A                                    ______________________________________                                    

Conditions: 2/1 H2/CO, 500-2000 SHSV feed rate, 175°-225° C. reactiontemperature, 20+ hours on stream time.

Example 5 - Comparative performance of SiO₂, Al₂ O₃ and TiO₂ at 5.6atmospheres

Cobalt/alumina and cobalt/titania catalysts were prepared by theincipient wetness technique using aqueous acetone on methanol cobaltnitrate solutions. The surface areas were silica - 280 m² /gm, alumina -180 m.sup. 2/gm, and titania (60% rutile) - 10 to 20 m² /gm. The saltwas decomposed in oxygen and reduced in flowing hydrogen until all ofthe metal was reduced. Samples were taken after achieving steadystate, >24 hours on stream, from a fixed bed reactor at 200 ° C., 2/-hydrogen:Co, 516 atmospheres.

    ______________________________________                                                     Activity, Moles CO Converted                                     Catalyst     g-atom CO/Hour                                                   ______________________________________                                        19% Co/Al.sub.2 O.sub.3                                                                    3.2                                                              12% Co/TiO.sub.2                                                                           0.5                                                              ______________________________________                                    

The results show higher activity and productivity for cobalt/alumina v.cobalt/titania at relatively low pressure.

Example 6 - Comparative Performance of Al₂ O₃ and TiO₂ Supported Co-Reat 1 atmosphere

Examples as shown in European Patent Application 0 313 375.

    ______________________________________                                        Catalyst         % CO Conversion                                              ______________________________________                                        12% Co/1% Re/Al.sub.2 O.sub.3                                                                  33                                                           12% Co/1% Re/TiO.sub.2 *                                                                       17                                                           12% Co/1% Re/TiO.sub.2 **                                                                      17                                                           ______________________________________                                         *Support Calcined at 500° C.                                           **Support Calcined at 600° C.                                     

Conditions: H₂ pretreatment at 1° C./minute to 350° C., hold for 10hours, 2/1 H₂ /CO, 1680 cm³ /g cat/hr feed rate, 10-30 hr on streamtime.

These examples show a marked advantage for Co-Re/Al₂ O over Co-Re/TiO₂at low pressure. The data reported also show that the preferred catalystCo-Re/TiO₂ is more active than Co-Re on supports such as chromia,zirconia, magnesia, silica-alumina, or silica.

Example 7 - Comparative Performance of Al₂ O₃ and TiO₂ Supported Cobaltat 10 atmospheres

These examples shown in Castier, D. et al ACS Publication 0097-6156/84,0248-0039, Catalytic Materials, p. 39 (1984).

    ______________________________________                                        Catalyst      % CO Conversion                                                 ______________________________________                                        5% Co/Al.sub.2 O.sub.3                                                                      17                                                              5% Co/TiO.sub.2                                                                             23                                                              ______________________________________                                    

Conditions: H2 pretreatment at 480° C. for 0.5 hours, 3/1 H₂ /CO, 260°C. reaction temperature, 0.5 hours on stream time.

At ten atmospheres the advantage is reversed and Co/TiO₂ shows greaterCO conversion than Co/Al₂ O₃.

Example 8 - Comparative Performance of Al₂ O₃ and TiO₂

Supported Co-Re Catalysts at 20 atmospheres

Example 7c and 7d are taken from U.S. Pat. No. 4,568,663, Table I;Examples 7a and 7b were run at 200° C., 20 psig. GHVS =1000, H₂ /CO=2.0-2.15 and similarly as reported in U.S. Pat. No. 4,568,663.

    ______________________________________                                        Catalyst          % CO Conversion                                             ______________________________________                                        a) 12% Co/Al.sub.2 O.sub.3                                                                      41                                                          b) 12% Co/0.5% Re/Al.sub.2 O.sub.3                                                              63                                                          c) 12% Co/TiO.sub.2                                                                             65-75                                                       d) 12% Co/0.5% Re/TiO.sub.2                                                                     80-85                                                       ______________________________________                                    

Conditions: Calcination at 250° C., H₂ pretreatment as per Example 1 inU.S. Pat. No. 4,568,663, 2/1 H₂ /CO, 1000 SHSV gas feed rate, 200° C.reaction temperature, >24 hours on stream time.

These examples again show the advantage reversed and Co-Re/TiO₂ isbetter at converting CO than Co-Re/Al₂ O₃ at 20 atmospheres. However,because of greater surface area, the alumina catalysts can acceptgreater amounts of cobalt and rhenium, thereby considerably reducing theadvantage.

What is claimed is:
 1. A two stage hydrocarbon synthesis processcomprising:(a) reacting in a first stage, hydrogen and carbon monoxidein the presence of a supported cobalt or ruthenium catalyst andobtaining a CO conversion to C₂ + hydrocarbons of at least 50% atreaction conditions including a pressure of at least 10 atmospheres; (b)recovering a reaction product comprising hydrogen and carbon monoxideand separating liquid therefrom; (c) reacting in a second stage, at apressure below 10-12 atmospheres and no greater than the outlet pressureof the first stage, the remaining reaction products in the presence of acatalyst comprising catalytically effective amounts of cobalt supportedon alumina at reaction conditions; and (d) wherein the second stagecatalyst is at least as active for CO conversion to C₂ + hydrocarbons asthe first stage catalyst at second stage reaction pressure.
 2. Theprocess of claim 1 wherein the second stage catalyst contains rhenium.3. The process of claim 1 wherein the second stage conversion of CO toC₂ + hydrocarbons is at least about 50%.
 4. The process of claim 1wherein the first stage pressure is at least 13 atmospheres.
 5. Theprocess of claim 1 wherein the first stage catalyst comprises a metalselected from the group consisting of cobalt and ruthenium supported ona material selected from the group consisting of silica, alumina,silica-alumina, and titania.
 6. A two-stage hydrocarbon synthesisprocess comprising:(a) reacting in a first stage hydrogen and carbonmonoxide in the presence of a supported cobalt containing catalyst,converting at least about 50% of the CO to C₂ + hydrocarbons atreactions including a pressure of at least 15 atmospheres; recovering areaction product comprising hydrogen and carbon monoxide and separatingliquids therefrom; (c) reacting the remaining reaction products in asecond stage at a pressure below about 10 atmospheres and no greaterthan the outlet pressure of the first stage, in the presence of acatalyst comprising catalytically effective amounts of cobalt, a rheniumpromoter and an alumina support, covering at least about 50% of the COto C₂ + hydrocarbons at reaction conditions. and (d) wherein the secondstage catalyst is at least as active for CO conversion to C₂ +hydrocarbons as the first stage catalyst at second stage reactionpressure.
 7. The process of claim 6 wherein the CO partial pressure atthe second stage outlet is at least about0.3 atmospheres.
 8. The processof claim 6 wherein the second stage catalyst contains about 5 to 60 wt%cobalt and the rhenium:cobalt ratio is at least about 0.05:1.
 9. Theprocess of claim 6 wherein the first stage CO conversion is about60-70%.
 10. The process of claim 6 wherein the hydrogen:carbon monoxideratio of the first stage is about 1.7:1 to about 2.1:1.
 11. The processof claim 6 wherein the first stage catalyst contains a promoter selectedfrom the group consisting of ruthenium, rhenium, hafnium and cerium. 12.The process of claim 11 wherein the promoter is rhenium.
 13. The processof claim 1 wherein the second stage catalyst support is at least 50 wt%alumina.
 14. The process of claim 6 wherein the second stage catalystsupport is at least 70 wt% alumina.
 15. The process of claim 1 whereinthe second stage catalyst is more active for CO conversion to C₂ +hydrocarbons than the first stage catalyst at second stage operatingpressure.
 16. The process of claim 6 wherein the second stage catalystis more active for CO conversion to C₂ + hydrocarbons than the firststage catalyst at second stage operating pressure.